How would I something to the effect of
$\displaystyle f(x)=x+x+x+x+...+x$
and put an under brace under the x+x+x+x+x+...+x part that says "an even number of terms}?
Another tip: for best results, the ellipsis should be written using \dots = $\displaystyle \dots$ rather than ... = $\displaystyle ...$. And when the ellipsis is between two operators, use \cdots to get a proper center-alignment: e.g. $\displaystyle 1 + \frac12 + \frac13 + \cdots + \frac1n$ or $\displaystyle f(x) = \underbrace{x + x + x + x + \cdots + x}_{\text{An even number of terms}}$ (doesn't that look nicer?).
And thank you o_O for making me realize that there is a [noparse] tag. I used to resort to such trickery as writing [[i][/i]math]...[/tex]. This makes it much easier!